1. Field of the Invention
The present invention relates to a method of collecting impurities on the surface of a semiconductor wafer for collecting a tiny amount of impurities existing on the surface of the semiconductor wafer and in a thin film formed on the semiconductor wafer and analyzing the impurities.
2. Description of the Prior Art
In a conventional type manufacturing process of a semiconductor wafer, it is required to advance a process, always monitoring a contaminated state of the surface of the semiconductor wafer. Particularly, when contamination due to metal impurities occurs, the increase of leakage current and the deterioration of withstand pressure are caused and the reliability of a device is largely damaged. Then, to investigate the extent of contamination, inspection that metal impurities are collected from the surface of a semiconductor wafer, are chemically analyzed and the type and quantity of the metal impurities are specified is made. At this time, there is also a measuring method by total reflection fluorescent X-ray spectroscopy (TXRF) in which metal impurities are not required to be collected, however, as this method has an element such as Al and Na which cannot be analyzed or an element which is not satisfactory in sensitivity, a method of collecting and chemically analyzing is required.
For the method of collecting metal impurities, as disclosed in Japanese published unexamined patent application No. Hei5-283498 or No. Hei5-82495 for example, a method of dripping liquid to be collecting liquid on the surface of an inspected semiconductor wafer, scanning this droplet touching the droplet to the surface of the semiconductor wafer with the droplet kept spherical utilizing surface tension, incorporating metal impurities existing on the surface of the semiconductor wafer into the droplet and withdrawing the droplet is used.
Referring to process drawings shown in FIG. 4, an example of the conventional type collecting method will be described below. FIGS. 4A, 4B and 4C show a collecting process viewed from the top and from the side.
First, metal impurities existing on the surface of a semiconductor wafer to be inspected are dissolved by the vapor of hydrogen fluoride (HF) (a VPD process) and the surface of the semiconductor wafer is turned into a hydrophobic state. Next, as shown in FIG. 4A, the semiconductor wafer 1 is horizontally fixed on a rotating table 4 using a vacuum chuck, a syringe 3 made of Teflon (a registered trademark) is touched to the surface of the semiconductor wafer 1 from the upside so that the syringe is perpendicular to the surface of the semiconductor wafer 1 and collecting liquid is dripped from the end of the syringe 3 to the center of the semiconductor wafer 1. At this time, suitable space is left between the end of the syringe 3 and the surface of the semiconductor wafer 1 to prevent the syringe and the semiconductor wafer from coming in contact, a dripped droplet 2 is kept spherical by the end of the syringe 3, keeping surface tension and the syringe 3 supports the droplet 2. For the collecting liquid, liquid such as HF, solution acquired by mixing HF and H2O2 and pure water which is touched to the surface of the semiconductor wafer in a hydrophobic state and can acquire sufficient surface tension is used. As described above, the droplet 2 of collecting liquid dripped from the syringe 3 is put between the syringe 3 and the semiconductor wafer 1 as it is and the spherical shape is maintained by the surface tension.
Next, as shown in FIG. 4B, the droplet 2 draws a spiral locus from the center toward the periphery, coming in contact with the surface of the semiconductor wafer 1 in a spherical state and is scanned by turning the semiconductor wafer 1 with the center as the center of turning in a state in which the droplet 2 is put between the semiconductor wafer and the syringe and simultaneously moving the syringe 3 in the radial direction (in a direction shown by an arrow F) of the semiconductor wafer 1 as it is. As a result, metal impurities existing on the whole surface of the semiconductor wafer are incorporated in the droplet without exception. After the scan is finished, the droplet 2 is attracted and withdrawn by the syringe 3 this time as shown in FIG. 4C. Operation from dripping the collecting liquid till withdrawing it is often executed using an automated apparatus, however, the operation is also made manually. In that case, a silicon substrate chip (split into a square 2 to 3 cm long) is used to withdraw the droplet, for example, the droplet 2 is rolled on the surface of the semiconductor wafer 1, drawing a locus shown in a plan in FIG. 5 and is withdrawn.
However, the conventional type collecting method has the following problems. A first problem is that a collecting process takes very long time. For the reason, the amount of the collecting liquid is set to 0.5 ml or less, that is, a small value to collect metal impurities at as high sensitivity as possible, therefore, the size of the droplet becomes small and area in which the semiconductor wafer and the collecting liquid are touched is reduced, scanned distance is extended by the quantity and it takes much time to scan.
As the size of the droplet is small, the surface tension cannot be kept unless the droplet is scanned at low speed, the droplet is split in pieces and it becomes difficult to collect all liquid. In a manual case, the droplet may drop from the edge of the semiconductor wafer in withdrawing the droplet. Further, when scanning and withdrawing time is extended, contamination from ambient atmosphere and the volatilization of collecting liquid may be caused and as the diameter of the semiconductor wafer is increased, the problems have been serious.